The Hubble Space Telescope is limping after a mechanical failure

Two of NASA’s premiere space telescopes, Hubble and Kepler, are currently out of commission — sad, if not entirely surprising, news for astronomers who depend on data from NASA’s aging fleet.

The 28-year-old Hubble went into temporary safe mode on Friday after detecting a mechanical failure with one of its gyroscopes — the spinning instruments that keep the telescope pointed steadily toward its targets.

Meanwhile Kepler, the powerhouse planet hunter that has detected some 4,000 new planets since it launched in 2009, has been in sleep mode since Sept. 26 to preserve dwindling fuel before its next data dump.

Both telescopes are nearing the ends of storied careers in space.

NASA stopped servicing Hubble in 2009, shortly before ending the shuttle program. Two of the six gyroscopes installed during the 2009 repair mission have already broken down, and the one that recently stopped working had been exhibiting what NASA called “end of life behavior” for about a year. It was no surprise when the slumping device stopped working a few days ago.

But then the backup gyro didn’t kick into action, creating a “very stressful weekend” for staff at NASA’s Goddard Space Flight Center and the Space Telescope Science Institute, deputy mission head Rachel Osten said on Twitter. All astronomy work is on hold while researchers attempt to figure out what is wrong.

If they aren’t able to get the “problematic” device up and running again, Hubble will revert to reduced gyro mode, using only one of its two remaining gyroscopes at a time. This mode would limit where Hubble can point but prolong the overall mission.

At one point, NASA had hoped that Hubble would stay in the sky long enough to observe in concert with the James Webb Space Telescope, a gold-plated Goliath that will be capable of capturing the oldest light in the universe. But repeated budget snafus and human failures have delayed the Webb telescope’s launch by more than a decade; it is now not expected to launch until at least 2021. NASA’s current operations contract for Hubble ends that year, though optimists say the spacecraft could last into the 2030s.

Many of NASA’s top space telescopes are more than 10 years old. The Chandra X-ray observatory, Spitzer Space Telescope, Swift Observatory and Fermi Gamma-ray Space Telescope have all exceeded the planned duration of their original missions by at least five years.

The end for Kepler is even nearer. The spacecraft has already operated for more than double the length of its original 3.5-year mission — functioning even after the second of four reaction wheels that keep it oriented was lost in 2013. It’s long been expected to run out of propellant sometime this year, but zero gravity makes it hard to measure how much fuel is left in the spacecraft’s tank.

“It’s like trying to decide when to gas up your car. Do you stop now? Or try to make it to the next station?” Kepler system engineer Charlie Sobeck wrote in a blog post this year. “In our case, there is no next station, so we want to stop collecting data while we’re still comfortable that we can aim the spacecraft to bring it back to Earth.”

The recent decision to put the telescope in sleep mode is intended to preserve its remaining fuel until Kepler can make contact with the Deep Space Network, the global system of antennae through which spacecraft communicate with Earth. When Kepler’s allotted DSN time begins Oct. 10, it will switch on and beam back data on more than 30,000 stars and galaxies in the constellation Aquarius collected during its most recent 27-day observing campaign.

There is no guarantee that the spacecraft will be able to transmit the science data.

“The fact that we managed to collect data in light of Kepler’s low fuel pressure is yet another incredible achievement by our engineers,” the mission’s guest observer office tweeted last week. “If successful, it would be an unexpected bonus.”

Once NASA decides to close out the mission, engineers will command the spacecraft to turn off its transmitters — preventing “pollution” of the airwaves. Then the spacecraft will be allowed to drift, alone in the dark.

Its wide, Earth-trailing orbit around the sun means that it will fall farther behind our planet until Earth effectively “laps” it, giving the spacecraft a gravity boost that slings it forward until it nearly catches up with Earth from behind. This graceful gravitational dance may continue indefinitely, with Kepler never coming closer than the moon, until the sun expands into a red giant and engulfs the inner solar system or some other cosmic phenomenon intervenes.

Like Kepler, TESS is designed to scan the sky in the visible part of the light spectrum, but its specialty is finding planets around bright, nearby stars. Researchers anticipate that it may discover 10,000 worlds during its mission.

“Kepler broke open the field in a rather dramatic way,” TESS principal investigator George Ricker said before the launch in April. “But TESS is opening an entirely new window on the universe.”